Recovery of acids



Feb. 18, 1941.

E. M. FRANKEL ETAL RECOVERY OF ACIDS Filed May 7, 1958 2 Sheets-Sheet l m f INVENTORS EDWARD M Pram/ 54 ARTHUR POLL/{K ATTORNEY Patented Feb. 18, 1941 UNITED STATES PATENT OFFICE 2,231,898 RECOVERY or ACIDS Application May 7, 1938, Serial No. 206,564

20 Claims.

Our invention relates to the recovery of organic acids from their corresponding salts. In carrying out our invention we decompose the salt with an acid which is normally gaseous and whose activity can be varied by pressure to be greater or less than the activity of the acid being recovered. Further conditions to be satisfied are that the released acid be soluble in the liquefied gaseous reactant, and that the'salt which is to be decomposed be relatively insoluble therein. A typical example of the improved method is the recovery of aceticacid from sodium acetate by the use of carbon dioxide wherein the liquefied C02 reacts with the sodium acetate in the presence of suflicient water to liberate acetic acid which immediately dissolves in the excess liquefied CO2 and is thereby removed from the zone of the reaction by removal of the excess liquefied C02. The latter is then brought to a distilling zone and the C02 distilled off, leaving the acetic acid to be recovered. Sodium bicarbonate is formed in the reaction zone and is from time to time removed.

Many attempts have been made to recover acetic acid from the corresponding sodium, calcium or other salts by the use of liquefied carbon dioxide. Thus, Ipatiew, Journal of Russian Physical Chemical Soc., vol. 45, page 992 (1913), subjected acetate solutions to the action of C02 under a pressure of about 800 pounds at about 90 C., and reported a recovery of 1 gram calcium carbonate from 50v cc. of a 12% solution of calcium acetate after seven days. Others have worked with liquid carbon dioxide in organic solvents, e. g., Woodruff et al., U. S. Patent 1,946,419.

Our present invention differs from the foregoing essentially in that we have succeeded in getting the reaction to go substantially to completion by taking advantage of the solubility of the released acid in .the liquefied acid gas to remove the same substantially as fast as it is formed. By proceeding in this manner we have been able to get nearly quantitative yields of acetic acid, for example, from its sodium and calcium salts. In recovering the gaseous reactant by distillation, it will be noted that in the usual case where the reactant is a gas at room temperature and pressure, the liquefied material may be handled close to its critical temperature, which means that the latent heat of the substance is extremely low, so that very little heat would be needed to evaporate off the liquefied gas from the acid to be recovered; and by the same reasoningnot much cooling will be required to convert the vapor of the acid gas back into liquid. It is evident that such considerations when availed of make for highly economical operation of the. process.

Our invention also includes recovery of acids in dilute or crude state by a cycle of reactions including the foregoing, in which the acid to be recovered is first neutralized by a salt of the gaseous reactant, with recovery of the salt formed of the acid whose recovery is sought and of the gas liberated, followed by decomposition of the recovered salt by the gaseous reactant in liquefied form. This results in the recovery of the original acid in relatively pure and concentrated form and of the salt of the'gaseous reactant which is now used to recommence the cycle.

Our invention will be best understood by reference to the following detailed description taken with the annexed drawings in which Fig. 1 shows an arrangement of apparatus suitable for recovering acetic acid from acetate salts.

Fig. 2 shows an arrangement of apparatus providing for a continuous method of recovery of acetic acid from its salts.

In carrying out the invention sodium acetate, for example, is placed in the extractor I, Fig. 1. The form of the'sodium acetate may vary, as will be pointed out hereinafter; preferably solid crystals are used having an average water content corresponding to the formula NaCzHzOaHzO to NaCzHsOzBI-IzO. The system is then pressured by carbon dioxide from the cylinder II by opening valve l2. When the pressure has reached the maximum as denoted by the gauge l3, say 700-1000 lbs. per square inch depending upon the temperature, the valve I2 isclosed and valve I4 is opened permitting liquid CO2 from the inverted cylinder l5 to flow to the apparatus and fill the same to some such level as shown in the sight glass [6. Valve I4 is then closed and liquid CO2 is caused to flow from pipe [8 (in the manner to 40 be subsequently explained) through valves l9 and through the material in the extractor l which is maintained at a desired temperature by means of water in the jacket 2|. The flow of liquid C02 is continued until the same now bear- 45 ing some acetic acid by reaction of the sodium acetate in the extractor I reaches the boiler 22 through pipe 23. Boiler 22 is provided with jacket 24 through which is circulated hot water entering through pipe 25 and leaving through pipe 26. As soon as liquid CO2 has reached boiler 22 distillation is started by the heat from the water in the jacket 24, the CO2 vapor leaving the boiler through pipe 21. and valve 28 and passing upwardly throughthe riser 30 to T 3|. Riser 30 being closed;

I through pipe 14'.

preferably has a jacket 32 through which the water from pipe 26 leading from jacket 24 circulates, such water leaving through pipe 33.

Valve 34 being closed and valve 35 open, the CO2 vapor now passes through condenser 36 formed of concentric inner and outer pipes within the outer of which flows cooling water from pipe 38. The CO2 is condensed in the inner coil, passes out of the condenser through pipe 18 having valve 40 and sight box 4|. Pipe l8 also has a jacket 42 through which cooling water passes from pipe 43 before it passes to pipe 38 connect-v ing jacket 42 and outer pipe of condenser 36.

Because of the head of CO2 liquidin the pipe I8 above the boiler 22, positive circulation-of liq-'- uid CO2 takes place through extractorl. Pipe 18 thus also serves as a reservoir of liquid extractant. When extraction is complete the boiler 22 'is. depressured by opening valve 34 and closing valve 45 in the line 23 and valve 35 following the cooler 36. After theexcesspressurehas been removed, the acetic acid is drained through the valve 41.

In carrying out the invention according to Fig. 2 sodium acetate, for example, from container 5| *is mixed in mixer 52 with a mediumwhich is preferably a saturated solution of sodium'acetate fed from. the tank filthy meansofrpipe 54. Any othersuitablemedium or vehicle may be employed'which does not interfere with the reaction, as for example, kerosene or other hydrocarbon' liquid. The slurry produced is'then fed to the'-extra1ctor'56- through pipe 51, valve 510. beingl o'pen. This extractor may be of various types; a preferred one:of which is a filter press, as for exampleofthe sweetland'or'leaf type, consisting of a set of filter leaves housed in a pressure type shell. The' sodium'a'cetate slurry is allowed to build-up on theleaves 58 of the filter press, the filtered liquid leaving the extractorthroughpipe '59'by means of which itpasses back to tank 53 aided by pump 59a, valve 5% being open. When a filter cake of satisfactory dimensions has been built up in the extractor 56'va1ves'5'la'and 5% are closed whereupon liquid r carbon dioxide from the receiver'lz is introduced into the extractor (externally of leaves '58) through pipe 66. The liquid CO2 reacts-with the sodium acetate in extractor 56 bysimple double decomposition, as already mentioned,

'forming aceticfacid which is dissolved in the liquid e'xtractant, leaving behind the-decomposition product, sodium bicarbonate. The extractant n'o'wfiowsout of the extractor 56 through pipe 61* to boiler 63; valve B'la'being open and valve 801) means'of steam' coil BS'to vaporizeoff the liquid extractant, leaving the acetic or other organicacid behind in the boiler '68. The-gaseous vapors leave the boiler through pipe H1, arecon'densed in condenser H and are collected in receiver 12 from which the" liquid extractant again passes to the extractor 56; The 002 may be replenished to the boiler 68' as needed from the tank 13 After the suitable concentranon of acetic acid has built upin the boiler 68, it is removed therefrom through pipe 15 andcollectcd in receiver 1%. Also, the mixture of sodium bicarbonate formed and unreacted upon sodium acetate may be removed, preferably as slurry, by back washing the filter 56 with liquor (containing mostly sodium bicarbonate) from the tank 19, which liquor passes through pipe 80 to the manifold 8 I, the resulting slurry passing through pipe 82 to a filter 83 preferably of the Oliver or rotary type, the filtrate being pumped back through pipe Here suffi'cientheat is provided by 84 by pump 85 either to manifold 8| through pipe 85 or directly to tank 79, the appropriate valves shown, Ma and 83b, being manipulated as required. The solid product of the reaction, sodium bicarbonate, containing some unreacted upon sodium acetate is delivered from filter 83 into hopper 81.

In decomposing the alkali metal salts with carbon dioxide-with liberation of the corresponding acid, one molecule of water is required for each molecule of acid produced in accordance witlrthe equation In reactingwith calcium salt one molecule of water produces-two molecules of acetic acid, the reaction being as follows:

Therefore, it is necessary to have a certain amount .of waterpresent. However, water greatly in excesso-f the theoreticalquantity cuts down the yield. The best results have been obtained using;frormllllltov 300% or more water based upon the amount theoreticallyrequired. Sodium acetate. occurs. in the, form ,of NaC2H3Oz.3I-I 2O and satisfactory results have. been obtained v by usingthis quantity of water as well as that correspondingto NaCzHaOzHzO. This salt does not occur naturally, but, the present purpose is served by mixingthe. quantities of anhy rous sodium acetate and NaCzHsOzBI-EO in amounts togive NaCzI-IsOaI-hQ. In, carrying out the reaction upon sodium acetate 3H2Owith liquid CO2 we have had a yield. of 85% acetic acid in a run lasting fivev hours Working with liquid, CO2 a pressure is necessary of 60071000 pounds, the temperatultebeingkept slightly below the critical temperature of 31 C. The particle, size ofthe sodium:acetate-,mayv be 80, mesh or finer, good resultsv-beingihad with the smaller mesh sizes providecl, there. is suflicient head, of, the liquidCOz to afford the desired rapidity of circulation. The strength of acid. recovered is as high as, 95%. Substantiallygthe same, resultsare to be had using, calciumtacetate;

The-use of sulfur dioxide instead of carbon dioxide wasfound; to give. equal y. good if not superior results with-the advantage that lower pressuresare. involved, since the working, pressure-ofnsulfcurdioxide maybe taken as around 100 lbs. per square inch as against as high as 1000- lbs. per square inch for carbon dioxide. The corresponding. reaction is as follows:

The advantage. of using a normally weak acid as the active agent is, first, the reaction product is relatively valuable, e. g., sodium. bicarbonate, sodium bisulfite; and secondly, salt so obtained can in many instances beused to recover the acid which is sought in, concentrated form. For example, sodiumbicarbonate may under certain conditions be used, to recover acetic acid itself with liberationof carbon; dioxide, which in turn may-be recoveredand'used in the cycle. Thus, in the distillation of wood,,theso called pyroligneous liquor obtained is neutralized with sodium bicarbonate, the solution evaporated either to dryness or to a point where sodium acetate will crystallize out and the sodium acetate'decomposedinto 'sodium bicarbonate and acetic acid in themanner described. This recovery cycle of acetic acid recovery may profitably be employed even though the-sodium a-cetate is not quantita:

tively decomposed, any remaining sodium acetate being added along with the sodium bicarbonate to a fresh supply of pyroligneous liquor. In a similar manner the present invention may be applied to other sources of acetic acid. Similarly, solutions of acetic acid may be neutralized with sodium bisulfite, with recovery of sodium acetate and sulfur dioxide, which thereupon may be liquefied and used to liberate relatively pure concentrated acetic acid with formation of sodium bisulfite in the manner above described.

In contrast to the end products obtained by the use of CO2 and S02, for example, those obtained by the use of the heavy or strong acids, such as sulfuric, phosphoric, hydrochloric, are of relatively little value. Thus, if sodium acetate is decomposed with sulfuric acid, an end product is sodium sulfate or sodium acid sulfate, which is of relatively little value, the same being true of sodium phosphate and sodium chloride.

We believe that the principle of our invention is of broad application to the recovery of acids of varied nature. The first requisite is that the liquefied reactant under the conditions of use by a stronger acid than the acid whose recovery is sought. Thus, liquefied CO2 is insufliciently active to decompose the alkali formates, whereas such reaction is readily accomplished with liquefied S02. acid and those higher in the series being less active than acetic acid, are readily'decomposed by liquefied C02. The only other conditions which must be met for the reaction to succeed are (2) the released acid be soluble in the liquefied acid gas, and (3) the resulting salt of the reaction be relatively insoluble in the liquefied acid. Among other acids or acid substances having the requisite ionization constants to serve as reactants may be mentioned hydrogen cyanide, HCN, hydrogen sulfide, HzS, etc.

In the claims, alkali forming metal is to be understood to cover both the alkali metals and the alkaline earth metals. It will be understood that various changes in the method of carrying out our invention will occur to those skilled in the art, without, however, departing from the spirit of our invention or from the scope of the appended claims.

tion of CO2 in liquid form under pressure in which the acid to be liberated is soluble, removing the liquefied CO2 containing the dissolved liberated acid, and recovering said acid from said liquefied C02.

2. In a method of deriving a carboxylic organic acid from a salt thereof, the steps which consist in subjecting said salt in the presence of sufilcient water to furnish the water of reaction, to the action of CO2 in liquid form under pressure in which the acid to be liberated from said salt is soluble, continuously removing the liquefied CO2 containing the dissolved liberated acid, and recovering said acid. Y

3. In a method of deriving acetic acid from the acetates of the alkali forming metals, the steps which consist in subjecting such acetates in the presence of sufficient Water to furnish the water of reaction, to the action of liquefied CO2 at a temperature lower than the critical temperature for CO2 and under a pressure corresponding to a vapor pressure of the liquefied CO: at such tem- On the other hand, salts of propionic perature, removing the liquefied CO2 containing the dissolved liberated acetic acid and recovering said acid by distillation of the acetic acid-enriched liquefied C02.

4. In a method of deriving acetic acid from the acetates of the alkali forming metals, the steps which consist in subjecting such acetates in the presence of sufiicient water to furnish the water of reaction, to the action of liquefied S02, removing the liquefied S02 containing the dissolved liberated acetic acid and recovering said acid by distillation of the acetic acid-enriched liquefied S02.

5. The method of deriving acetic acid from acetates, which consists in conveying a solid alkali metal acetate in the form of a slurry in a liquid vehicle to the reaction zone in which said solid material is retained, said solid material containing not substantially more water than is necessary to furnish the water'of reaction; subjecting said material to the action of a stream of liquefied CO2, removing said liquefied CO2 together with the released acetic acid dissolved therein, separating said liquefied CO2 from the acetic acid with recovery of acetic acid, and removing the solid reaction products in said reaction zone with recovery of the corresponding salt of reaction.

6. The method according to :claim 5, in which said liquid vehicle after it has conveyed solid alkali metal acetate to the reaction zone, is used to convey fresh quantities of solid alkali metal acetate to be decomposed tosaid zone.

7. The method which comprises reacting in aqueous solution an acid (A) and a salt of an acid formed in water of a normally gaseous acid substance (B) which substance when dissolved in water at atmospheric pressure has a weakly acid reaction which is weaker than that of acid (A), but which in liquefied form has sufficient acid reactivity .to decompose salts of acid (A), recovering from the above stated reaction a corresponding salt of acid (A) with liberation of acid su stance (B), reacting said recovered salt of acid (A) with acid substance (B) in liquefied form under pressure and in the presence of I sufiicient water to maintain the reaction, under which conditions the activity of acid substance (B) is suftlcient to liberate acid (A), removing from the zone of reaction acid substance (B) in liquefied form with acid (A) dissolved therein, and separating acid (A) from acid substance (B).

8. The method according to claim 7 in which acid A is neutralized with an alkali forming metal salt of acid substance B.

9. In a method of deriving a free acid from a salt thereof, the steps which consist in subjecting said salt in the presence of sufficient water to maintain the reaction, to the action of a gaseous acid substance in the liquefied state in which the acid to be liberated is soluble and the salt formed by decomposition is relatively insoluble, which substance when dissolved in water at atmospheric pressure has a weakly acid reaction but which in liquefied form has sufficient acid reactivity under the stated conditions to decompose said salt and liberate the acid thereof, removing from time to time during the course of the reaction from the zone of reaction the liquefied gas containing the released acid in solution, and recovering therefrom said released acid.

10. In a method of deriving a free acid from a salt thereof, the steps which consist in subjecting said salt in the presence of water in an amount to maintain the reaction, but insufiicient to retard the reaction substantially, to the action of a gaseous acid substance in the liquefied state'in which the acid tobe liberated is soluble and the salt formed by decomposition is relatively insoluble, which substance when dissolved in water at atmospheric pressure has a weakly acid reaction but which in liquefied form has sufficient acid reactivity under the stated conditions to decompose said salt and liberate the acid thereof, removing during the course of the reaction from the zone of reaction the liquefied gas containing the releasedacid in solution, and recovering therefrom said released acid.

11. In a method of deriving a carboxylic organic acid from a salt thereof, the steps which consist in subjecting said salt in the presence of sufiicient water to maintain the reaction, to the action of a gaseous acid substance in the liquefied state in which the acid to be liberated is soluble and the salt formed by decomposition is relatively insoluble, which substance when dissolved in Water at atmospheric pressure has a weakly acid reaction but which in liquefied form has sufficient acid reactivity under the stated conditions to decompose said salt and liberate the acid thereof, removing from time to time during the course of the reaction from the zone of reaction the liquefied gas containing the released acid in solution, and recovering therefrom said released acid.

12.' In a method of deriving a carboxylic organic acid from the corresponding salt of an alkali formingmetal, the steps which consist in subject ing said salt in the-presence of sufiicient water to furnish the water of reaction, to the action of S05 in liquid form Lmder pressure in which the acid to be liberated from said salt is soluble, continuously removing the liquefied S02 containing the dissolved liberated acid, and recovering said acid.

13} The method of deriving acetic acid from acetates, which consists in conveying a solid alkali metal acetate in the form of a slurry in a liquid vehicle to the reaction zone-in which said solid material is retained, said solid material containing not substantially more water than is necessary to furnish the water of reaction, subjecting said'material to the action of a stream of liquefied S02, removing said liquefied S02 together with the released acetic acid dissolved therein, separating saidliquefied S02 from the acetic acid with recovery of acetic acid,'and removing the solid reaction products in said reaction zone with recovery of the corresponding salt of reaction.

14. The method according to claim 13, in which said liquid vehicle after it has conveyed solid alkali metal acetate to the reactionzone, is used to convey fresh quantities of solid alkali metal acetate to be decomposed to said zone.

15. The method which comprises reacting in aqueous solution an acid (A) with a salt of .an acid formed 'by the presence in water of C02, thereby recovering the corresponding salt of acid A with theliberation of 002,- reacting said salt of acid A with COzin liquefied form under pressure in the presence of sufficient water to maintain the reaction, under which condition the activity of the CO2 is sufiicient to liberate acid A, removing from the zone of reaction the C02 in liquefied form with acid A dissolved therein, and separating acid A from said liquefied CO2.

16. The method according to claim 15 in which in the first step acid A in aqueous solution is neutralized with an alkali forming metal salt'of carbonic acid. 7

17. The method which comprises reacting in aqueous solution an acid (A) with a salt of an acid formed by the presence in water of S02, thereby recovering the corresponding salt of acid A with the liberation of S02,'reacting said salt of acid A with S02 in liquefied form under pressure in the presence of sufficient water to maintain the reaction, under which condition the activity of the S02 is sufilcient to liberate acid A, removing from the zone of reaction the S02 in liquefied form with acid A dissolved therein, and separating acid A from said liquefied S02.

1.8. The method'according to claim 1'7 in which in the first step acid A in aqueous solution is neutralized with an alkali forming metal salt of an acid formed by dissolving S02 in water.

-19. In a method of deriving acetic acid from acetates of thealkali forming metals, the steps which consist in subjecting such acids in the presence of water in an amount between one and not substantially in excess of three mcls per mol of acetate; to the-actionoflique-fied 002 at a temperature lower than the critical temperature for CO2 and under a pressure corresponding to a vapor pressure of the liquefied CO2 at such temperature, removing the liquefied C02 containing the dissolved, liberated acetic acid, and recovering such acid by distillation of the acetic acidenriched liquefied C02.

20. In a method of deriving acetic acid from acetates of the alkali forming metals, the steps which consist in subjecting such acetates in the presence of water in an amount between one and not substantially in excess of three mols per mol of acetate, to the action of liquefied S02, removing the liquefied S02 containing the dissolved acetic acid, and recovering said acid by distillation of the acetic acid-enriched liquefied S02.

EDWARD M. FRANKEL. ARTHUR POLLAK. 

